Clusters tetrahedral geometry

In the families of heptanuclear clusters, two geometries are found the capped octahedron that is typical for 98-valence electrons, and the vertex-sharing open tetrahedral (butterfly) stmctures typical for 106-valence electrons. An example of the former is Osy(CO)22 (51) an example of the latter is [H2AuOsg(CO)2Q] (52). In the AuOs cluster anion, the gold atom is at the vertex-sharing position. 

There is only one example of a pentanuclear cluster, namely [Au5(dppm)3(dppm-H)](N03)2 (622), where the metal core adopts a spiked-tetrahedral geometry of gold atoms. 3... 

The most stable cluster consists of an aggregation of four cation vacancies in a tetrahedral geometry surrounding an Fe3+ ion, called a 4 1 cluster. Cations in the sodium chloride structure normally occupy octahedral sites in which each metal is coordinated to six nonmetal atoms. The central Fe3+ ion in the 4 1 cluster is displaced into a normally unoccupied tetrahedral site in which the cation is coordinated to four oxygen ions. Because tetrahedral sites in the sodium chloride structure are normally empty, the Fe3+ is in an interstitial site. Equation (4.1) can now be written correctly as... 

According to the E.A.N. rule, all clusters of four atoms possessing the tetrahedral geometry require a total of 60 valence electrons. A tetrahedron with six two-electron/two-center bonds corresponding to the... 

The solid state structures of methylzinc fert-butyl sulfide , methylzinc isopropyl sulfide and ethylzinc ethyl sulfide have been determined. All are aggregated clusters. Methylzinc tert-butyl sulfide is a pentamer (MeZnSBu-t)5 in which four of the sulfide groups are /r -bonded to four zinc atoms and one sulfide is /r -bonded between two zinc atoms (Figure 108). Consequently, four of the zinc atoms have a distorted tetrahedral geometry while the remaining zinc atom is trigonal planar. 

Fig. 29. The bicapped tetrahedral geometry established for the Os6(CO)18 metal cluster. For reasons of clarity the carbonyl ligands have been omitted from the illustration.

Examples of Gold Heteronuclear Cluster Compounds Exhibiting Capped Tetrahedral Geometries... 

The electronic properties of Ag4 as well as its ionized forms have been examined in detail by CNDO and EH, as shown in Table IV. Both procedures predict the linear form to be the stable neutral cluster, but as the cluster loses electrons the tetrahedral geometry becomes more stable. This is because the symmetric molecular orbitals are lower in energy for the tetrahedral than linear geometry and only these would be occupied as the cluster loses electrons. These effects are in accord with the electron spin resonance (ESR) experiments of Eachus and Symons (41) on the cationic forms of Ag4 clusters in frozen... 

A bond elongation occurs when electrons are added to the neutral cluster. This occurs particularly for the tetrahedral geometry but also, for the linear geometry. The monoanions of Ag are calculated to be stable with the exception of Ag2 (i.e., the reaction 3 Ag- Ag2" + Ag+ requires energy). It is calculated that Ag2" is unstable by 0.5 eV, but Ag3" is stable by 1.33 eV. 

In virtually all its stable compounds carbon forms four bonds and has coordination numbers of 2 (=C— or =C=), 3 (=CQ, or 4, with linear, triangular (planar), and tetrahedral geometries, respectively CO has coordination number 1. In interstitial carbides (Section 7-3), certain metal cluster compounds1 (Section 7-9), and very stable trigonal bipyramidal and octahedral penta- and hexa(aurio)methanium cations of the type (LAu)5C+ and (LAu)6C2+, where L is a phosphine,2 carbon atoms are found with coordination numbers of 4, 5, or 6. Coordination number 5 is also found in compounds with bridging alkyls such as Al2Me6, in some carboranes (Section 5-12), and in reactive carbocations.3... 

Besides the tetrahedral geometry, four-atom clusters are known with butterfly (16-XVII) and planar (16-XVIII) structures as well as still others. 

Fig. 12. Schematic diagram of metal binding by human CCS. hCCS domains 1, 2, and 3 are labeled with roman numerals. Cysteine residues are designated as S. The disulfide bond in domain 2 is indicated by S-S. (a) Cobalt binding to hCCS. Electronic absorption spectra indicate that two equivalents of Co(II) bind per hCCS monomer, one through three or four cysteine residues in a tetrahedral geometry, and one with a geometry similar to that found in the zinc site of SODl (see text) (Zhu et al., 2000). (b) Copper binding to hCCS. XAS indicates that two Cu(I) ions bind per hCCS monomer in a sulfur-only liganding environment, with an additional heavy atom scatterer peak suggesting the presence of a /t2-bridged dicopper cluster (Eisses et al., 2000).

In terms of its coordination chemistry, the silver(I) ion is typically characterized as soft . Although originally believed to only bind ligands in a linear arrangement, it was soon shown that it can adopt a variety of coordination environments, the most common one being a four-coordinate tetrahedral geometry. Square-planar complexes are not rare, and various silver(I) cluster complexes also contain three-and five-coordinate sUver(I) ions. 

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